A ZOOLOGY expert at a Welsh university has urged caution following a plan by British scientists to manufacture an “alien invasion” to kill off Japanese knotweed.

The knotweed (Reynoutria japonica), which has beguiling, creamy-white flowers, was brought to Britain from the Far East by Victorian explorers.

But it quickly turned into a highly invasive, virtually unstoppable pest, its stems able to break through concrete.

Even when chopped into bits, parts of the Japanese knotweed turn themselves into whole new plants.

In the Swansea area, one of the worst affected parts of Britain, its total biomass is said to exceed 62,000 tonnes.

But despite the problems, Professor Paul Brain, chair of zoology at Swansea University, is unhappy with plans by the Commonwealth Agricultural Bureaux International (Cabi) to introduce an “alien parasite” to help curtail its spread.

The parasite in question is a Japanese jumping plant louse or psyllid which attaches itself to plants and sucks out their sap.

But fears have been raised that this solution could itself bring its own problems to Wales’ native habitat.

Prof Brain said: “Psyllids are already causing problems in Ireland where they were introduced along with eucalyptus trees so there are concerns that the parasite might attack other, native UK plants.

“Japanese knotweed damages our natural habitats and it has been estimated that the eradication of this clone plant by traditional means would cost £1.6bn so, getting an insect to do it, seems commercially attractive.

“Given the history of biological control disasters, however, one has to be a bit worried.”

The list of alien parasite experiments which went wrong include:

Killer snails (Euglandina rosea) and flat worms (Platydemus manokwari) were introduced into Hawaii to control the giant African land snail, but then started attacking native snails too;

The cane toad was brought in by Australian farmers to control the cane beetle but it spread rapidly into a “plague” and it was found the poison-squirting toads killed animals who tried to attack them, including pet dogs and cats;

The mongoose was introduced to Hawaii to keep down rat numbers but the creatures found it much easier to dine on birds eggs instead;

And a form of weevil (Rhinocyllus conicus) was introduced to North America to control musk and Canadian thistles, but it also had a ravenous taste for native thistles.

However, scientists have successfully used Trichogramma ostriniae, a small wasp, to successfully control the European Corn Borer which devoured high value crops in the US.

And the cactus moth has been used successfully in Australia to control the prickly pear cactus.

Also, one of the earliest “biological control” experiments, in the late 1800s, was a success.

In the US the citrus industry was threatened when the Cottony Cushion Scale (a plant eating bug) was discovered.

However, the Vedalia Beetle and a parasitoid fly were introduced to control the pest successfully.

Dick Shaw, principal investigator at Cabi, said Japanese Knotweed was having such a devastating effect on landscapes as well as pavements and buildings, something had to be done.

He said: “Japanese knotweed has been described as having the biodiversity value of concrete. It just smothers the ground in a mass.

“We hope the psyllid will get this plant under control.”

Cabi scientists have also been busily testing the Japanese jumping lice on more than 100 native plants and important crops that grow in Britain.

So far they have found the British plants are not affected.

Chris Rostron, of the British Wildlife Trusts, warned: “Knotweed poses a serious threat to Britain's biodiversity but introducing any non-native species carries risks and must be thoroughly researched.”